Method of monitoring a tandem connection in a multi-protocol label switching (MPLS) telecommunication network

ABSTRACT

It is disclosed a method of monitoring, a Multi-Protocol Label Switching network, a tandem connection of a Label Switched Path. The tandem connection to be monitored has an input node and an output node. The method comprises the following steps: receiving a first Operation Administration Management packet at the input node; and generating a second OAM packet at the input node. According to the invention, the following steps are further performed: a tandem connection header is inserted into the first OAM packet; both the first OAM packet with the tandem connection header and the second OAM packet are sent towards the output node; and the second OAM packet is identified according to the presence or absence of said tandem connection header.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. Ser. No.12/930,621, filed Jan. 12, 2011, and assigned to the same assignee asthe current application, which in turn is a continuation application ofU.S. Ser. No. 11/483,086, filed Jul. 10, 2006, and assigned to the sameassignee as the parent application.

FIELD OF THE INVENTION

The present invention relates to the telecommunication field. Moreparticularly, the present invention relates to a method of monitoring atandem connection in a Multi-Protocol Label Switching telecommunicationnetwork. The invention further relates to network nodes which areadapted to implement such a method and to an MPLS network comprisingsuch nodes.

This application is based on and claims the benefit of Italian PatentApplication no. MI2005A001570 filed on Aug. 12, 2005 and European PatentApplication no. 06075897.6 filed on Apr. 18, 2006, which areincorporated by reference herein.

BACKGROUND OF THE INVENTION

In packet-switched telecommunication networks, user data are dividedinto packets, each packet being routed from a source node to atermination node through a path comprising a plurality of intermediatenodes.

The path each packet follows from the source node to the terminationnode can be dynamically determined hop-by-hop, as it happens forinstance in IP (Internet Protocol) networks.

Alternatively, the path of a packet may be determined before the packettransmission. This usually happens in MPLS (Multi-Protocol LabelSwitching), which are defined by the Request for Comments RFC3031,January 2001.

In MPLS networks, the path defined between the source node and thetermination node is termed Label Switched Path (briefly LSP in thefollowing description).

In a MPLS network, the source node of an LSP path assigns to each packetto be transmitted a switching header comprising a first label, and itsends the packet to the first intermediate node of the determined LSPpath. The first intermediate node, according to the value of such afirst label, sends the packet to the second intermediate node of the LSPpath, after replacing the first label with a second label which can beread by the second intermediate node. And so on, until the packetreaches the termination node, which removes the packet header and itprocesses the user information comprised therein.

Therefore, source nodes and termination nodes are adapted to create andremove the switching header comprising the label, respectively. On theother hand, intermediate nodes are adapted to replace the labelcomprised into the switching header of the received packet with adifferent label which can be read by the following intermediate node.

In MPLS networks, it is known to monitor an LSP path by means of a setof functions which are termed OAM (Operation, Administration andManagement). More particularly, such OAM functions are adapted to checkthe path integrity, the transmission performance along the path, or thelike. The OAM functions in MPLS networks are standardized by the ITU-TRecommendation Y.1711, February 2004.

According to this Recommendation, the OAM functions are performed bytransmitting particular packets along the LSP path to be monitored,which are termed OAM packets, Such OAM packets are periodicallytransmitted from the source node to the termination node along the LSPpath to be monitored.

Each OAM packet comprises, besides the switching header, an OAM header,whose label is equal to a predefined value (14, according to the ITU-TY.1711), for distinguishing OAM packets from user packets. Such OAMheader is placed after the switching header. Further, the payload ofsuch OAM packets comprises a field, which is termed TTSI (TrailTermination Source Identifier), which in turn comprises two fieldsLSR-ID and LSP-ID of sixteen and four bytes, respectively, whichcomprise the identifier of the LSP path source node and the identifierof the LSP path, respectively.

When an operator transmits user data from a source node to a terminationnode, it can be useful to evaluate transmission performance both alongthe entire LSP path, and along one or more sections thereof. Generallyspeaking, a section of a path (i.e. the sequence of two or moresucceeding nodes) is termed “tandem connection”.

Monitoring a tandem connection is particularly interesting when, forinstance, a path between a source node and a termination node of anetwork operator comprises one or more tandem connections which aremanaged by other operators. Indeed, in case of faults or receptionerrors, it is important, for the operator which is responsible for thetransmission along the entire path, to determine whether the fault/erroroccurred on a network section within its own competence or within thecompetence of another operator. Also the operator which is responsiblefor the tandem connection is interested in monitoring its own tandemconnection.

Therefore, there is the need, for an operator, to have the capability ofmonitoring simultaneously both an entire path and tandem connections ofsuch a path.

In particular, in the MPLS network field, Huawei Technologies Co. Ltd,in the document entitled “Proposal for MPLS administrative domain”,submitted at ITU-T SG13 Plenary Meeting, COM13D104-E (Apr. 25^(th)-May6^(th) 2005) proposed to monitor a section of an LSP path in a MPLSnetwork as follows. In each domain, the input node of the domain mayinsert a so-called “per-domain OAM packet” into the LSP containing thetandem connection to be monitored. The “per-domain OAM packet” usessimilar format with the packet for monitoring the whole LSP, as definedby the above cited ITU-T Y.1711. However, in this “per-domain OAMpacket”, the TTSI field should be modified to the identifier of theinput node of the tandem connection to be monitored. The output node ofthe tandem connection to be monitored thus monitors the value of theTTSI field, so that it can distinguish OAM packets for monitoring thewhole LSP and “per-domain OAM packets”.

This solution exhibits some drawbacks. First of all, such a solution isnot compliant with the current MPLS apparatuses, as it requires aprocessing of the payload (in particular, the TTSI field) of thereceived OAM packets. Moreover, such a processing, even if it could beperformed by the current MPLS apparatuses, would require the use of asignificant amount of resources at each node, thus subtracting resourcesavailable for transmitting user packets.

SUMMARY OF THE INVENTION

Therefore, the general object of the present invention is providing amethod of monitoring a tandem connection in a MPLS network whichovercomes the aforesaid problems.

In particular, an object of the present invention is providing a methodof monitoring a tandem connection in a MPLS network which can beimplemented by current MPLS apparatuses, which requires a lower amountof resources in comparison with known methods, and which is quicker.

These and other objects are achieved by a method as set forth in therespective independent claims. Further advantageous features are setforth in the respective dependent claims. All the claims are deemed tobe an integral part of the present description.

In a first aspect, the present invention provides a method ofmonitoring, in a Multi-Protocol Label Switching (MPLS) network, a tandemconnection of a Label Switched Path (LSP). The tandem connection to bemonitored has an input node and an output node. The method comprise thefollowing steps: receiving a first Operation Administration Management(OAM) packet at the input node, and generating a second OAM packet atthe input node. According to the invention, the following steps arefurther performed: a tandem connection header is inserted into the firstOAM packet; the first OAM packet, with the tandem connection header, andthe second OAM packet are sent to the output node; at said output node,the second OAM packet is identified according to the presence or absenceof the tandem connection header.

Preferably, the tandem connection header is inserted in the first OAMpacket after its switching header.

According to an advantageous embodiment, a predefined value is writtenin an experimental use field of the tandem connection header.

Preferably, a predefined value is written in a label field of the tandemconnection header.

According to a second aspect, the present invention provides a networknode of an MPLS network, wherein the network node is an input node of atandem connection to be monitored. The network node is adapted toreceive a first OAM packet to be sent to an output node of the tandemconnection to be monitored, and to generate a second OAM packet to besent to the output node. The node further comprises means for insertinginto the first OAM packet a tandem connection header so that the secondOAM packet can be identified at the output node of the tandemconnection.

According to a third aspect, the present invention provides a networknode of an MPLS network, wherein the network node is an output node of atandem connection to be monitored. The network node is adapted toreceive a first OAM packet generated upstream an input node of thetandem connection to be monitored and a second OAM packet generated atthe input node. The node further comprises means for identifying saidsecond OAM packet according to the presence or absence of a tandemconnection header inserted at the input node.

According to a fourth aspect, the present invention provides an MPLSnetwork comprising a tandem connection to be monitored, wherein thetandem connection comprises an input node as set forth above and anoutput node as set forth above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more clear by reading the followingdescription, given by way of example and not of limitation, to be readwith reference to the accompanying drawings, wherein:

FIG. 1 schematically shows an MPLS network comprising three domains;

FIG. 2 schematically shows a first example of the method of monitoring atandem connection applied to the network of FIG. 1, according to thepresent invention;

FIG. 3 schematically shows a tandem connection header;

FIG. 4 schematically shows a second example of the method of monitoringa tandem connection applied to the network of FIG. 1, according to thepresent invention; and

FIG. 5 schematically shows a third example of the method of monitoring atandem connection applied to the network of FIG. 1, according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 schematically shows an exemplary MPLS network. Such an MPLSnetwork comprises nine nodes 1, 2, . . . 9. For simplicity, the othernodes of the MPLS network are not shown in FIG. 1.

The nodes 1, 2, 3 are comprised in a first domain D1; the nodes 4, 5, 6are comprised in a second domain D2; the nodes 7, 8, 9 are comprised ina third domain D3. The domains D1, D3 are managed by a first operator A,while the second domain D2 may be managed either by the first operator Aor by a second operator B, as it will be shown in greater detail hereinafter.

In FIG. 1, it is assumed that the node 1 has to transmit user data tothe node 9. The node 1 is the source node of the LSP path through whichuser packets are sent to node 9. Such an LSP path, shown in FIG. 1,comprises the cascade of the nodes 1, 2, 3 . . . 9.

As the source of the LSP path, node 1 inserts a first label to packetsto be sent to node 2, indicated in FIG. 1 as packets U1. Therefore, eachpacket U1 includes a header H1 and a payload UP comprising user data, asschematically shown in FIG. 1. The header H1 of each packet U1comprises, in addition to the label, other data, which are not furtherdescribed since they are not relevant to the present description. Thenode 1 sends the packet U1 to the node 2.

The node 2, upon reception of the packet U1 from the node 1, replacesthe first label comprised into the header H1 with a second label whichis adapted to be processed by the node 3, thus forming a header H2, andit sends the so obtained packet U2 to the node 3. The node 3, uponreception of the packet U2 from the node 2, replaces the second labelcomprised into the header H2 with a third label which is adapted to beprocessed by the node 4, thus forming a header H3, and it sends the soobtained packet U3 to the node 4. And so on, until the node 9, whichreceives from the node 8 a packet U8 with a header H8, it removes theheader H8 and it processes the user payload UP.

It is now assumed that the first operator A wishes to monitor both thewhole LSP path of the user packets, and the tandem connection TCcomprising the nodes 4, 5, 6, which is indicated by a double arrow inFIG. 2. For instance, in case the domain D2 is managed by the firstoperator A itself, the operator A may wish to monitor the tandemconnection TC, the domain D2 being particularly critical (e.g., veryhigh traffic flows, valuable traffic, or the like). In case the domainD2 is managed by a second operator B, the first operator A may wish tomonitor the tandem connection TC for checking whether possible failuresoccur on network sections of its own or on network sections of thesecond operator B.

FIG. 2 schematically shows a first example of the method of monitoringthe tandem connection TC applied to the network of FIG. 1 according toan embodiment of the present invention, under the assumption that alsothe domain D2 is managed by the first operator A.

The first operator A inserts a first flow of OAM packets, which will betermed herein after “flow of tandem connection OAM packets”, along theLSP path through the input node (i.e. the first node) of the tandemconnection to be monitored. Further, the operator A extracts such a flowof tandem connection OAM packets from the LSP path through the outputnode (i.e. the last node) of the tandem connection to be monitored. Forinstance, in FIG. 2, the input node of the tandem connection TC is thenode 4, whilst the output node of the tandem connection TC is the node6.

Therefore, the flow of tandem connection OAM packets goes through thewhole tandem connection TC to be monitored.

In the following description, the expression “path OAM packets” willindicate a flow of packets generated by the source node of an LSP pathfor monitoring a whole LSP path, which are different from the abovecited tandem connection OAM packets.

The input node 4 of the tandem connection TC is adapted to insert atandem connection header in any packet (either user packet or path OAMpacket) received from the node 3, which precedes it in the LSP path.Similarly, tandem connection output node 6 is adapted to remove a tandemconnection header from any packet (either user packet, or path OAMpacket, or tandem connection OAM packet) received from the node 5, whichprecedes it in the LSP path.

The Applicant has noticed that, for performing such operation ofinserting a tandem connection header in a packet so that such a packetis still compliant with the MPLS standard even after the insertion ofsuch a header, it is advantageous to employ a “label stacking”mechanism, which is provided by the MPLS standard, as described by theRequest for Comments RFC3032, January 2001.

According to such a RFC3032, an MPLS packet may comprise more than oneheader, each header comprising a respective label. Each header (and eachlabel comprised therein) corresponds to an LSP path of a differentlayer. More external headers correspond to higher path layers, whilemore internal headers correspond to lower path layers. Each node, forperforming label switching, always processes the most external header.Each header comprises, in addition to the label, a stack bit, whichindicates whether the current header is followed or not by a furtherheader. In this manner, an arbitrary number of headers may be stacked.

Therefore, according to the above mentioned RFC3032, the input node froma lower layer path to a higher layer path inserts in front of eachreceived packet a higher layer header, which temporally precedes thealready existent lower layer header. Intermediate nodes of the higherlayer path process the more external header, i.e. the higher layerheader. The output node of the higher layer path removes the higherlayer header, so that the following nodes, which belong to the lowerlayer path, process the most external header, i.e. the lower layerheader.

However, according to the present invention, differently from the labelstacking according to the above mentioned RFC3032,the input node of thetandem connection inserts such a tandem connection header after thealready present switching header H1, H2, . . . , H8.

Therefore, the packet structure is compliant with the label stackingmechanism as defined by the RFC3032, January 2001. Moreover, as eachnode of the LSP path performs switching by processing the most externalheader, the switching mechanism is unchanged.

FIG. 3 schematically shows the structure of a tandem connection headerTCH according to an embodiment of the present invention. Such a tandemconnection header TCH comprises a label field L, an experimental usefield EXP, a stack field S and a Time To Live field TTL. According to anembodiment of the present invention, the size of each field is compliantwith MPLS RFC3032, January 2001, i.e. the field L size is 20 bits, thefield EXP size is 3 bits, the field S size is 1 bit and the field TTLsize is 8 bits.

According to embodiments of the present invention, the tandem connectionheader TCH inserted in a packet preferably comprises, in the label fieldL, a tandem connection label having a predefined value. Such apredefined value should be different from the above cited predefinedvalue 14 of the label comprised in OAM headers,

According to other embodiments of the present invention, the tandemconnection header TCH inserted in a packet preferably comprises, in theexperimental use field EXP, a predefined value. Such a predefined valueshould be preferably different from “000”, which is the current standardvalue inserted in field EXP both of switching headers and of OAMheaders. Preferably, the label field L comprises the same predefinedvalue 14 which is currently used for OAM headers. Therefore, OAM headersand tandem connection headers only differ for their field EXP value.This advantageously allows to avoid reserving a label value for tandemconnection monitoring, thus preserving all available label values(except 14) for switching purposes.

By still referring to FIG. 2, such Figure shows respectively a flow ofuser packets (U1, U2, . . . , U8 on the upper row), a flow of path OAMpackets (P1, P2, . . . , P8 on the intermediate row), and a flow oftandem connection OAM packets (T4, T5 on the lower row), which areprocessed according to the present invention.

Reference can be initially made to the flow of user packets U1, U2, . .. , U8, which is generated by the node 1 and received by the node 9.

The node 4 receives the packet U3 from the node 3 and it calculates thenew switching header H4. As the node H4 is a tandem connection inputnode, according to the present invention, it inserts a tandem connectionheader TCH after the switching header H4. The node 4 therefore obtainsthe packet U4, which is sent to the node 5. The switching header H4 isthen the most external header in the packet U4. It is substantiallyunchanged relative to the header H3, except for the label value and thestack bit value, which is set to “1”, so that it indicates the presenceof a further header.

The node 5 receives the packet U4 from the node 4 and it calculates thenew switching header H5, thus obtaining a packet U5, which is sent tothe node 6.

When the node 6 receives the packet U5, it calculates the new switchingheader H6 and, as the node 6 is a tandem connection output node, itremoves the header TCH. Also in this case, the switching header H6 issubstantially unchanged relative to the switching header H5, except forthe label value and the stack bit value, which is set equal to “0”, sothat it indicates that there is no successive header.

According to embodiments of the present invention, the above describedprocessing is applied also to the flow of path OAM packets P1, P2, . . ., P8 generated by the node 1 for monitoring the whole LSP path betweenthe node 1 and the node 9.

Indeed, in FIG. 2 it can be observed that the path OAM packets P1, P2, .. . , P8 (shown on the intermediate row in FIG. 2) have switchingheaders H1, H2, . . . , H8 and TCH header which are identical to theones of the corresponding user packets U1, U2, . . . , U8. In additionrelative to the corresponding packets U1, U2, . . . , U8, such packetsP1, P2, . . . , P8 comprise the above cited OAM header, indicated inFIG. 2 as OAMH. Packets P1, P2, . . . , P8 then comprise an OAM payload,which is indicated in FIG. 2 as OAMP.

According to embodiments of the present invention, the value of thelabel comprised in the tandem connection header TCH is different fromthe value of the label comprised in the header OAMH.

Reference should now be made to lower row of FIG. 2. As alreadymentioned, the operator A, for monitoring the tandem connection TC,inserts along the LSP path a flow of tandem connection OAM packetsthrough the input node of the tandem connection TC, i.e. the node 4.Such tandem connection OAM packets are indicated in FIG. 2 as T4. Eachpacket T4 generated by the node 4 comprises the switching header H4, theheader OAMH, and a payload OAMP.

As the node 5 receives a packet T4 from the node 4, it calculates thenew switching header H5, but it does not insert any header TCH, sincesuch a packet T4 has been generated inside the tandem connection TC. Itthen sends the so obtained packet T5 to the node 6.

Therefore, the node 6 receives path OAM packets P5 having a header TCH,which are adapted to monitor the whole LSP path. On the other hand, thenode 6 receives tandem connection OAM packets T5 having no header TCH,which are adapted to monitor the tandem connection TC.

Therefore, the node 6, by checking the presence or the absence of theheader TCH in each received packet P5, T5, is capable of distinguishingpath OAM packets P5, to be forwarded to the node 7 for monitoring theLSP path, and tandem connection OAM packets T5, to be extracted from theLSP path and processed directly for monitoring the tandem connection TC.

More particularly, the node 6 forwards path OAM packets comprising a TCHheader to the node 7. It can be noticed that during this step the node 6also removes the header TCH of the path OAM packet, since the node 6 isthe output node of the tandem connection TC. Therefore, the node 9receives and processes OAM packets compliant with the standard, withoutany additional header.

On the other hand, the node 6 extracts from the LSP path tandemconnection OAM packets T5, which do not comprise any header TCH. Also inthis case, the node 6 processes OAM packets which are compliant with thestandard, without any additional header.

FIG. 4 shows a second example of the method of monitoring a tandemconnection according to an embodiment of the present invention, whereinthe domain D2 is managed by a second operator B.

Also in this case, the first operator A wishes to monitor the tandemconnection TC. However, in this case, the first operator A is notallowed to access nodes 4 and 6, and therefore it is not allowed toinsert and extract flows of tandem connection AOM packets through thenodes 4 and 6.

In this case, the first operator A identifies a second tandem connectionTC′, which comprises the tandem connection TC to be monitored and whichhave an input node and an output node accessible by the first operatorA.

For instance, the second tandem connection TC′ may be the tandemconnection 3-4-5-6-7 which in shown in FIG. 4.

Therefore, there are two nested tandem connections: a first tandemconnection TC, with input node 4 and output node 6 which are accessibleby the operator B, and a second tandem connection TC′ comprising thetandem connection TC, with input node 3 and output node 7 which areaccessible by the operator A.

In this case, the first operator A inserts on the LSP path themonitoring flow of the tandem connection TC through the input node 3, adit extracts it from the LSP path through the output node 7.

Moreover, each tandem connection input node (i.e. nodes 3 and 4) isadapted to insert a tandem connection header in any packet (user packet,path OAM packet, and tandem connection OAM packet) received from thenode which precedes it in the LSP path. Similarly, each tandemconnection output node (i.e. nodes 6 and 7) is adapted to remove atandem connection header from any packet (user packet, path OAM packet,and tandem connection OAM packet) received from the node which precedesit in the LSP path.

As already described with reference to FIG. 2, also in this case tandemconnection input nodes are adapted to insert the tandem connectionheader after the switching header already comprised in the receivedpackets.

For instance, FIG. 4 shows a flow of user packets (U1, U2, . . . , U8,upper row), a flow of path OAM packets (P1, P2, . . . , P8, intermediaterow) and a flow of tandem connection OAM packets (T3, . . . , T6, lowerrow), respectively, which are processed according to embodiments of thepresent invention.

Firstly, reference can be made to the flow of user packets U1, U2, . . .U8 generated by the node 1 and received by the node 9.

When the node 3 receives a packet U2 from the node 2, it calculates thenew switching header H3 and, as the node 3 is the input node of thetandem connection TC′, the node 3 inserts a tandem connection header TCHin the received user packet, whose label preferably has a predefinedvalue. Preferably, the header TCH is inserted after the header H3. Thenode 3 forwards the so obtained packet U3 to the node 4.

The node 4 receives the packet U3 from the node 3, and it calculates thenew switching header H4. Moreover, as the node 4 is the input node ofthe tandem connection TC, also the node 4 inserts a tandem connectionheader TCH in the received user packet. Such a header TCH is preferablyidentical to the one inserted by the node 3. Therefore, the user packetU4 sent by the node 4 and received by the node 5 will have two identicalconsecutive headers TCH, which are preferably placed after the header H4(which is still the most external header).

The node 5 receives the packet U4 from the node 4, and it calculates thenew switching header H5, thus obtaining a packet U5, which is sent tothe node 6.

When the node 6 receives the packet U5, it calculates the new switchingheader H6 and, as the node 6 is the output node of the tandem connectionTC, one of the headers TCH is removed from the user packet. Preferably,the header which is closer to the switching header is removed. In thisway, the stack bit value is kept unchanged. The node 6 then sends the soobtained packet U6 to the node 7.

When the node 7 receives the packet U6, it calculates the new switchingheader H7 and, as the node 7 is the output node of the tandem connectionTC′, the remaining header TCH in the packet U6 is removed. The soobtained packet U7 is then forwarded to the node 8, and then to the node9, wherein it is received and processed in a traditional way.

The above described processing also applies to the flow of path OAMpackets P1, P2, . . . , P8 generated by the node 1 for monitoring thewhole LSP path between the node 1 and the node 9.

Indeed, by referring to FIG. 4, it can be noticed that the path OAMpackets P1, P2, . . . , P8 have switching headers H1, H2, . . . H8 andtandem connection headers TCH identical to the corresponding userpackets U1, U2, . . . , U8.

Reference can now be made to the lower row of FIG. 4. As alreadymentioned, the operator A, for monitoring the tandem connection TC,inserts along the LSP path a flow of tandem connection OAM packetsthrough the input node of the tandem connection TC′, i.e. the node 3.Such packets are indicated in FIG. 4 as T3. Each packet T3 generated bythe node 3 comprises the switching header H3, the header OAMH and apayload OAMP.

When the node 4 receives a packet T3 from the node 3, it calculates thenew switching header H4 and, as the node 4 is the input node of thetandem connection TC, it inserts a tandem connection header TCH afterthe header H4, thus obtaining a packet T4.

The packet T4 is then forwarded to the node 5, which calculates the newswitching header H5, and it sends the so obtained packet T5 to the node6.

As the node 6 receives such a packet T5, it calculates the new switchingheader H6 and, as the node 6 is the output node of the tandem connectionTC, it removes the header TCH from the packet T5, thus obtaining apacket T6, which it is sent to the node 7.

The node 7 then receives path OAM packets P6 having a header TCH andbeing adapted to monitor the whole LSP path. On the other hand, the node7 of the operator A receives tandem connection OAM packets T6 having noheader TCH, and being adapted to monitor the tandem connection TC.

Therefore, the node 7, by checking either the presence or the absence ofthe header TCH in each received AOM packet P6, T6, is capable ofdistinguishing path OAM packets P6, to be forwarded to the node 8 formonitoring the LSP path, and tandem connection OAM packets T6, to beextracted from the path LSP and to be directly processed for monitoringthe tandem connection TC.

More particularly, the node 7 forwards to the node 8 path OAM packets,which comprise a header TCH. It can be noticed that during this step thenode 7 removes also the header TCH from the path OAM packet, the node 7being the output node of the tandem connection TC′. The node 9 thereforereceives and processes standard OAM packets, without any additionalheader.

On the other hand, the node 7 extracts from the LSP path tandemconnection OAM packets which do not comprise any header TCH, anddirectly processes them. Also in this case, the node 7 processesstandard OAM packets, without any additional header.

Therefore, the present invention has the following advantages.

First of all, the information allowing to distinguish path OAM packetsand tandem connection OAM packets, and thus allowing the operator A tomonitor at the same time both the whole path and the tandem connection,is a tandem connection header. Therefore, for distinguishing path andtandem connection OAM packets, each node simply has to consider theheader of the received OAM packets, without the need of processing theirpayload.

On one hand, this makes the method according to the present inventioncompatible with current MPLS apparatuses and, on the other hand, thisreduces both the amount of resources (memory, CPU, etc.) required forsuch an operation and the processing time.

A further reduction of processing complexity can be achieved by havingtandem connection headers identical for all the nodes and for all thepacket types.

Moreover, as the tandem connection header is inserted after theswitching header, switching mechanisms along the LSP path are unchanged.

Moreover, all the packets are processed by nodes in the same way, bothuser packets, and OAM packets. Also this advantageously helps inreducing processing complexity at each node, as each node is notrequired to distinguish different types of packets, but it simplyperforms the same operation on all the received packets.

Moreover, all the packets (user packets, path OAM packets and tandemconnection OAM packets) are received by the respective termination nodewhich has to process them without neither any additional header nor anyadditional payload field with respect to the standard format of an MLPSpacket. Advantageously, then, nodes performing packet processing, and inparticular payload processing, do not have not be modified with respectto current MPLS nodes.

Moreover, advantageously, the method according to the invention allowsthe first operator A and the second operator B to monitor substantiallyat the same time the LSP path section comprised between nodes 4 and 6,which is managed by the second operator B.

FIG. 5 shows a third example of the method of monitoring a tandemconnection applied to the network of FIG. 1 according to an embodimentof the present invention.

In the example of FIG. 5, it is assumed that the first operator A wishesto monitor both the whole LSP path between node 1 and node 9, and thetandem connection TC managed by the second operator B. At the same time,also the second operator B wishes to monitor its own tandem connectionTC.

In the following description, the expression “path OAM packets” willstill refer to OAM packets generated by the node 1 for monitoring theLSP path. The expression “tandem connection OAM packets A” will indicateOAM packets generated by the operator A for monitoring the tandemconnection TC. The expression “tandem connection OAM packets B” willindicate OAM packets generated by the operator B for monitoring thetandem connection TC.

Similarly to the example of FIG. 4, for monitoring the LSP path, theoperator A transmits along the LSP path from node 1 a flow of path OAMpackets, indicated as P1, P2, . . . , P8 in FIG. 5. Similarly, as in theexample of FIG. 4, the operator A inserts along the LSP path a flow oftandem connection OAM packets A through the node 3, per monitoring thetandem connection TC, which are indicated as TA3, . . . , TA6 in FIG. 5.

Moreover, in the example of FIG. 5, the operator B inserts along thepath LSP a flow of tandem connection OAM packets B through the node 4,for monitoring the tandem connection TC, which are indicated as TB4, TB5in FIG. 5.

All the three OAM packet flows and the user packet flow are processed byeach node according to the above operations, described by referring toFIGS. 2 and 4, i.e.:

-   -   each input node of a tandem connection (3 and 4) inserts a        tandem connection header TCH after the switching header H1 H2, .        . . , H8 of each received packet; and    -   each output node of a tandem connection (6 and 7) removes a        tandem connection header TCH from each received packet.

In FIG. 5, for simplicity, the user packet flow is not shown.

Therefore, by referring to the first row of FIG. 5 (P1, P2, . . . , P8)it can be noticed that the node 3 inserts a first header TCH in eachreceived packet P2, and it forwards the so obtained packet P3 to thenode 4. Similarly, the node 4 inserts a second tandem connection headerTCH in each received packet P3, and it forwards the so obtained packetP4 to the node 5. The node 5 receives the packet P4, it modifies itsswitching header and it forwards the so obtained packet P5 to the node6. The node 6 removes a header TCH from each received packet P5(preferably, the one closest to the switching header), and it forwardsto the node 7 the packet P6 so obtained. The node 7 extracts theremaining header TCH from each received packet P6 and it forwards to thenode 8 the packet P7 so obtained.

By referring now to the second row of FIG. 5 (TA3, . . . , TA6), thenode 4 inserts a TCH header in each received packet TA3, and it forwardsto the node 5 the packet TA4 so obtained. The node 5 receives the packetTA4, it modifies its switching header and it forwards the packet TA5 soobtained to the node 6. The node 6 removes the header TCH from eachreceived packet TA5, and it forwards the packet TA6 so obtained to thenode 7.

Finally, by referring to the last row of FIG. 5 (TB4, TB5), the packetsTB4 inserted by the operator B through the node 4 are generated insidethe tandem connection TC, which does not contain any further tandemconnection. Therefore, nodes do not insert further headers TCH to suchpackets.

Consequently, when the node 6 receives an OAM packet P5, TA5, TB5, itprocesses the headers of the received OAM packet. If the received OAMpacket does not include any header TCH, the node 6 recognizes such anOAM packet as a tandem connection OAM packet B, it extracts it from theLSP path and it processes it. On the other hand, if the received OAMpacket comprises at least one header TCH, the node 6 recognizes such anOAM packet either as a path OAM packet or as a tandem connection OAMpacket A, and it forwards (as packet P6 or TA6) to the node 7.

Therefore, when the node 7 receives an OAM packet P6 or TA6 from thenode 6, it processes the header of the received OAM packet. if thereceived OAM packet comprises a single header TCH, the node 7 recognizesthe OAM packet as a tandem connection OAM packet A TA6, it extracts itfrom the LSP path and it processes it. On the other hand, if thereceived OAM packet comprises two headers TCH, the node 7 recognizessuch a OAM packet as a path OAM packet P6, and it forwards it to thenode 8 as a packet P7.

It is obvious that the example of FIG. 5 may be extended to exampleswherein the LSP path of a first operator comprises a tandem connectionmanaged by a second operator, which in turn comprises a further tandemconnection which is managed by a third operator.

The invention may also apply to a situation wherein an LSP path of afirst operator comprises a plurality of cascaded tandem connections,which may be adjacent or non adjacent, each tandem connection beingmanaged by a different operator.

Similarly, the invention may also be applied to a situation wherein anLSP path of a first operator comprises a plurality of cascaded tandemconnections, which are all managed by a same second operator.

The invention claimed is:
 1. A method of monitoring, in a Multi-ProtocolLabel Switching network, both a label-switched path and a tandemconnection that is part of the label-switched path, the tandemconnection comprising a sequence of successive nodes, a first of whichis an input node and a last of which is an output node, the methodcomprising: receiving a first packet at the input node, the first packethaving a first switching header; processing at the input node the firstpacket to generate a modified version of the first packet that includeswithin the first packet a recalculated first switching header and atandem connection header; generating at the input node a second packethaving a second switching header, wherein the second packet is aduplicate version of the first packet that includes within the secondpacket the recalculated first switching header; sending towards theoutput node both the first packet and the second packet; at the outputnode, identifying the second packet, and not forwarding the secondpacket further on the label-switched path; and at the output node,removing the tandem connection header from the first packet.
 2. Themethod of claim 1, wherein the first packet is one of a first userpacket and a first Operation, Administration and Management (OAM) packethaving a first OAM header.
 3. The method of claim 1, wherein the methodfurther comprises the step of identifying a received packet at theoutput node as being the second packet based on the absence of thetandem connection header.
 4. The method of claim 1, wherein the methodfurther comprises the step of identifying a received packet at theoutput node as being the first packet based on the presence of thetandem connection header.
 5. The method of claim 1, wherein the firstpacket is identified for use in monitoring the label-switched path. 6.The method of claim 1, wherein one or more of the first switching headerand the second switching header comprises a label field carrying a labelrespectively calculated to be processed by the next node receiving thepacket.
 7. The method of claim 1, wherein one or more of the firstswitching header and the second switching header is re-calculated ateach intermediate network node to insert a new label adapted to beprocessed by the next node along the label-switched path.
 8. The methodof claim 1, further comprising the step of: inserting the tandemconnection header into the first packet.
 9. The method of claim 8,further comprising the step of: removing the tandem connection headerfrom the packet.
 10. The method of claim 1, wherein the label-switchedpath and the tandem connection are monitored substantially concurrently.